The rising percentage of parents opting out of at least one mandatory vaccination could be a major factor in the recent increase in whooping cough cases. That's according to a study, published in Pediatricstoday, based on data from New York state, where religious exemptions to vaccination requirements are enforced loosely enough to allow parents to opt out based on personal or philosophical beliefs about the drugs, Reuters explains.

Researchers tracked data from the state's Department of Health. They noticed that the proportion of religiously exempt kids, while still very small, had nearly doubled in the state: 23 in 10,000 to 45 in 10,000. And in counties with more than 1 percent of children under a religious exemption, whooping cough cases were higher: 33 out of every 100,000 kids, compared to 20 per 100,000 kids in counties with an exemption rate under 1 percent.

But there's more: because the current whooping cough vaccine is less effective than the original, even vaccinated kids in counties with more exemptions are more susceptible to the illness. Most vaccines rely on "herd immunity" to boost their effectiveness — if a certain percentage of the population is vaccinated, the disease can't spread. Different diseases have different thresholds, and while the New York whooping cough numbers seem pretty tiny, increasing vaccination exemptions aren't just a localized issue.